The invention relates to a magnetic drilling stand having a drilling unit, in particular for core hole drilling, having an electromagnet for fixing the magnetic drilling stand on a component, having a voltage source which can be coupled to the electromagnet, and having a controller for monitoring the holding force of the electromagnet, with the controller having a device for monitoring the current level of the electromagnet in order to derive from this a signal for an adequate magnetic holding force or for a fault state.
The invention furthermore relates to a method for monitoring the holding force of an electromagnet in a magnetic drilling stand such as this.
A magnetic drilling stand and a method such as this are known from JP 571 027 09 A (Patent Abstracts of Japan). In this case, the holding force of an electromagnet in a magnetic drilling stand is monitored by a magnet sensor device which contains a hole element, a constant current source, resistors and a comparator circuit. The comparator circuit has resistors and a computation unit, in order to monitor the magnetic holding force. When the electromagnet is switched on, the magnetic drilling stand is magnetically held on a component. If the magnetic holding force is not adequate, then a warning signal is output via the comparator circuit, as a result of which the drilling unit cannot be operated.
Although a monitoring circuit such as this allows the magnetic holding force in a magnetic drilling stand to be monitored reliably to a certain extent, the activation of the drilling unit is, however, dependent only on one specific threshold value being reached. This may not ensure adequate reliability in some cases, in different operating states.
Furthermore, U.S. Pat. No. 5,035,547 discloses a magnetic drilling machine in which a switch is provided which monitors any movement of the drilling stand on a component. This switch may be a mercury switch, a vibration switch or the like.
Although this also results in a certain level of safety for the situation in which the magnetic holding force of the magnetic drilling stand is inadequate, disconnection takes place only when a certain movement has actually already occurred. This is not considered to be adequate for all hazard states.
According to DE-AS-1 811 583, the circuit for drilling operation is interrupted in a magnetic drilling stand when no current is flowing in the DC circuit of the electromagnet. Switching on is possible only when current is flowing through the electromagnet.
A controller such as this is not considered to be adequate to ensure an adequate electromagnet holding force in all operation states, since all that is monitored is whether the electromagnet is or is not actually switched on.
According to DE 31 00 933 C2, in the case of a magnetic drilling stand, a switch which responds to the magnetic flux of the electromagnet switches the current for drilling operation off if the magnetic flux density falls below a specific level. A reed switch is used as a switch for this purpose. This is fitted in the vicinity of the electromagnet such that it is influenced when the magnet is energized.
Although, in principle, such monitoring of the magnetic flux results in increased safety, reed switches are, however, relatively expensive and susceptible to defects.